Many garments yellow over multiple wash wear cycles, reducing the aesthetic value of the garment. In order to maintain the white appearance shading dyes may be used. For main wash applications these are preferably blue or violet dyes of the acid, direct or hydrolysed reactive dye classes. A number of problems arise during use that are dependent on the class of dye.
Direct dyes build up over multiple washes, and this can lead to a strong blue or violet colour on the garment. To make this overshading acceptable lower level of dye must be used reducing the benefit.
Acid dyes have the advantage that they do not build up over multiple washes. However no single acid dye has been found that shows high deposition to cotton and gives a true blue or violet shade to the cloth. Many are too green in colour for optimum shading effects. Additionally many acid dyes that deposit to cotton also deposit on nylon and this leads to overshading of nylon after multiple washes.